1. Field of the Invention
The invention relates generally to a current-perpendicular-to-the-plane (CPP) sensor based on the giant magnetoresistance (GMR) effect that operates with the sense current directed perpendicularly to the planes of the layers making up the sensor stack, and more particularly to a scissoring-type CPP-GMR sensor with dual sensing or free layers.
2. Background of the Invention
One type of conventional magnetoresistive sensor used as the read head in magnetic recording disk drives is a sensor based on the giant magnetoresistance (GMR) effect. A GMR sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically copper (Cu). In one type of GMR sensor, called a “spin-valve”, one of the ferromagnetic layer has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and the other ferromagnetic layer has its magnetization direction “free” to rotate in the presence of an external magnetic field. With a sense current applied to the sensor and in the presence of an applied magnetic field, the rotation of the free-layer magnetization relative to the fixed-layer magnetization is detectable as a change in electrical resistance.
In a magnetic recording disk drive spin-valve read sensor or head, the stack of layers are located in the read “gap” between magnetic shields. The magnetization of the fixed or pinned layer is generally perpendicular to the plane of the disk, and the magnetization of the free layer is generally parallel to the plane of the disk in the absence of an external magnetic field. When exposed to an external magnetic field from the recorded data on the disk, the free-layer magnetization will rotate, causing a change in electrical resistance. If the sense current flowing through the stack is directed parallel to the planes of the layers in the sensor stack, the sensor is referred to as a current-in-the-plane (CIP) sensor, while if the sense current is directed perpendicular to the planes of the layers in the sensor stack, it is referred to as current-perpendicular-to-the-plane (CPP) sensor. A CPP-GMR spin-valve type of read head is described by A. Tanaka et al., “Spin-valve heads in the current-perpendicular-to-plane mode for ultrahigh-density recording”, IEEE Transactions on Magnetics, Vol. 38 (1): 84-88 Part 1, January 2002.
CPP-GMR sensors are susceptible to current-induced noise and instability. The spin-polarized bias or sense current flows perpendicularly through the ferromagnetic layers and produces a spin transfer torque (STT) on the local magnetization. This can produce continuous gyrations of the magnetization, resulting in substantial low-frequency magnetic noise if the bias current is above a certain level. This effect is described by J.-G. Zhu et al., “Spin transfer induced noise in CPP read heads,” IEEE Transactions on Magnetics, Vol. 40, January 2004, pp. 182-188. To maximize the signal and signal-to-noise ratio (SNR) in CPP-GMR sensors, it is desirable to operate the sensors at a high bias current density. However, the adverse effect of STT limits the bias current at which the sensors can operate. One proposal to alleviate this problem to some degree is to increase the magnetic damping of the ferromagnetic free layer, i.e., to increase the effective thermal coupling between the magnetization (spin-system) and that of its host lattice. The free layer with magnetization gyrations caused by STT will lose energy to the lattice faster than it can absorb energy from the bias current via STT. US 2006/0221512 A1, assigned to the same assignee as this application, describes a CPP-GMR sensor with an antiparallel free layer (AP-free) structure, i.e., two free layers with magnetizations oriented antiparallel across a Ru spacer layer, wherein one of the free layers includes a NiFeTb film for magnetic damping of the other free layer across the Ru spacer layer. However, magnetic damping of the free layer in a CPP-GMR sensor does not address the STT problem associated with the pinned layer. The pinned layer, which is exchange-coupled to an antiferromagnetic layer, is difficult to magnetically damp in the same manner as proposed for the free layer due to the necessity of maintaining high exchange coupling of the magnetic layer(s) to the pinned layer and/or high AP-coupling between the reference and pinned layers if an AP-pinned structure is used.
A type of CPP-GMR sensor has been proposed that does not have a ferromagnetic pinned layer, but instead has dual ferromagnetic sensing or free layers separated by a nonmagnetic spacer layer. In the absence of an applied magnetic field, the magnetization directions or vectors of the two free layers are oriented generally orthogonal to one another with parallel magnetization components in the sensing direction of the magnetic field to be detected and antiparallel components in the orthogonal direction. With a sense current applied perpendicularly to the layers in the sensor stack and in the presence of an applied magnetic field in the sensing direction, the two magnetization vectors change their angle relative to one another, which is detectable as a change in electrical resistance. Because of this type of behavior of the magnetization directions of the two free layers, this type of CPP-GMR sensor will be referred to herein as a “scissoring-type” of CPP-GMR sensor. In a scissoring-type CPP-GMR sensor, there is no need for a ferromagnetic pinned layer and thus no need for an antiferromagnetic pinning layer. However, a single layer of hard magnetic material at the back of the sensor, opposite the air-bearing surface, is used to bias the magnetization directions of the two free layers so that they are roughly orthogonal to one another in the quiescent state, i.e., in the absence of an applied magnetic field. Without the hard bias layer, the magnetization directions of the two free layers would tend to be oriented antiparallel to one another. This tendency to be oriented antiparallel results from strong magnetostatic interaction between the two free layers once they have been patterned to sensor dimensions, but may also be the result of exchange coupling between the free layers through the spacer. The scissoring-type of CPP-GMR sensor is described by Seigler, et al., “Current-perpendicular-to-plane multilayer sensors for magnetic recording”, IEEE Transactions on Magnetics, Vol. 39(3), May 2003, pp. 1855-1858, and in U.S. Pat. No. 7,035,062 B2. However, a scissoring-type CPP-GMR sensor is still susceptible to STT, which limits the bias current density and thus the sensitivity of the sensor.
In addition to STT-induced instability, a scissoring-type CPP-GMR sensor is susceptible to magnetic instability from a different source. The detected signal field is aligned collinearly with the bias field from the hard bias layer above the sensor, rather than orthogonally as in the case of a conventional CPP-GMR spin-valve type sensor with two hard bias layers on each side. In situations where the signal field is antiparallel to the bias field, the total applied field on the scissoring-type sensor is reduced in magnitude (in particular that originating at track edges), and the sensor is more susceptible to magnetic instability than a spin-valve type sensor where the total applied field on the sensor is never smaller than the hard bias field (which is strongest at the track edges). This generally makes the stabilization of the scissoring-type sensor more difficult compared to a spin-valve sensor.
What is needed is a scissoring-type CPP-GMR sensor that can operate at high bias current densities without magnetic instability.